In the fields of optical communications or optical measurements, optical modulators such as intensity modulators having a Mach-Zehnder type optical waveguide are widely used. The Mach-Zehnder type optical waveguide has a configuration in which an input waveguide is branched into two branch waveguides and the two branch waveguides are merged into an output waveguide. Depending on the types of optical modulators, there are various types of Mach-Zehnder type optical waveguides such as a type using a single Mach-Zehnder type optical waveguide and a type in which another Mach-Zehnder type optical waveguide is assembled into each branch waveguide of a single Mach-Zehnder type optical waveguide in a nest shape.
When light waves propagating in the branch waveguides of the Mach-Zehnder type optical waveguide merge in in-phase in a merging portion, light waves output to an output waveguide are in an ON state. When the light waves merge in out-phase, the light waves are radiated into a substrate in which the optical waveguide is formed or are introduced into radiated-light waveguides disposed to interpose the output waveguide therebetween and the output of the output waveguide is in an OFF state. Hereinafter, an light wave output from the output waveguide in the ON state is referred to as an ON light and an light wave radiated from the merging portion in the OFF state is referred to as an OFF light or a radiated light.
Since the intensity variation of a light output from the Mach-Zehnder type optical waveguide exhibits sinusoidal characteristics, a modulation signal to be applied to a modulation electrode for modulating an optical waveguide propagating in the Mach-Zehnder type optical waveguide needs to be set to an appropriate operation bias point so as to obtain the light intensity of an output light depending on the application of the optical modulator.
Accordingly, in the related art, a part of an output light (ON light) derived to an optical fiber connected to the optical modulator or an OFF light is detected as a monitoring light by an optical receiving element such as an optical detector and the intensity state of the output light of the optical modulator is monitored. The operation bias point of the modulation signal to be applied to the modulation electrode is adjusted (bias-controlled) on the basis of the value (monitoring output) detected by the optical receiving element.
Even when the bias control is performed using the monitoring output as described above, there is a need that an output function of an optical fiber output and a monitoring output of the optical modulator should have a proportional or a complementary relationship with respect to the voltage applied to the modulation electrode and there should not be a bias shift therebetween, in order to optimize the output of the optical modulator. Accordingly, a structure for preventing unnecessary lights from being mixed into the monitoring light or a structure using two OFF lights has been proposed.
In the control of optical communications according to the related art, even when slight bias point misalignment occurs in the monitor output, it does not cause any particular problem. This is because the optical level detected as a signal is the maximum transmission or minimum transmission level of the output function of the intensity modulator having the Mach-Zehnder type optical waveguide. In this case, since the waveform is shaped on the basis of nonlinearity of the output function, several percent of bias shift is allowable.
On the contrary, with the recent increase in communication capacity, when a multi-level modulation format such as a differential quadrature phase shift keying (DQPSK) format is used, for example, it is necessary to set the bias point so that the quadrature point of the output function is the output light level. In this case, since the bias point is set to a point sensitive to the variation in light intensity, it is necessary to precisely control the operation bias point of the optical modulator, for example, with an accuracy of 1% or less of a half-wavelength voltage Vπ so as to keep the quality of the output signal good.
In a Y branch structure of the merging portion as the structure of the Mach-Zehnder type optical waveguide, when light waves are input in in-phase to the merging portion from two branch waveguides, most of the light waves are changed to a fundamental-mode light of the output waveguide and are output as the ON light. However, some of the light waves are radiated as a conversion loss proportionally as the ON light to both sides of the output waveguide.
When light waves are input in out-phase to the merging portion from the two branch waveguides, the output waveguide is designed to guide only the fundamental-mode light and thus lights having different phases (out-phase) are radiated as the off lights to both sides of the output waveguide. As a result, since some of the lights (varying in the same phase as the ON light) as the conversion loss in addition to the OFF lights (out-phase) are mixed, two radiated lights are not in out-phase but have a bias shift departing from the complementary state.
Accordingly, in a configuration for detecting only one radiated light as a monitoring light as described in Japanese Laid-open Patent Publication No. 2001-281507, since a state departing from the regular phase of the OFF light is detected, it is difficult to accurately adjust the bias to the quadrature point.
When a thin-plate structure with a thickness of 20 μm or less is used to enhance the performance of the optical modulator, it is necessary to dispose a radiated-light waveguide in the merging portion as described in Japanese Laid-open Patent Publication No. 2010-237376. In this case, since unnecessary lights do not diffuse but propagate in the substrate due to the thin-plate substrate having characteristics of slab waveguide, various unnecessary lights are likely to be mixed into the monitoring output and a bias shift is likely to occur between the main output which is the output of the optical modulator and the monitoring output.
As described in U.S. Pat. No. 6,795,620, it has been proposed to improve monitoring characteristics by using two radiated lights radiated to both sides of the output waveguide as monitoring lights. In this case, since the departure in phase difference between the radiated lights causes different signs between the monitoring outputs, the departure can be corrected using both radiated lights.
However, as described in U.S. Pat. No. 6,795,620, it is necessary to use an optical detector with a large optical receiving surface or two optical detectors in order to obtain a monitoring output. In the former, the optical detector with a large optical receiving diameter causes an increase in component size. In addition, there is a problem in that the fast frequency responsiveness of the monitoring output degrades. In the latter, the number of components increases and thus the structure or the connection is complicated, thereby causing an increase in size or an increase in cost.